1. Field of the Invention
The present invention relates to a telescope used for observation of an object and a compensation optical system in the telescope.
2. Description of the Related Art
An astronomical telescope is known, as an astronomical telescope for astronomical observation, which includes a primary mirror reflecting and imaging light beam from astronomical objects, a refractive optical system arranged at a principal focal point of the primary mirror or in the vicinity thereof, and an image pickup element photoelectrically converting an image formed.
In astronomical observation other than the zenith, the imaging position for light varies from color to color due to atmospheric dispersion, so that a deviation occurs in an observed star image due to a difference in the wavelength of light. In order to compensate degradation in optical performance due to such atmospheric dispersion, there is known an astronomical telescope in which an aberration compensation system (principal focal point compensation optical system) is provided at a focal point of the primary mirror or in the vicinity thereof (U.S. Pat. No. 6,038,068).
In the principal focal point compensation optical system disclosed in U.S. Pat. No. 6,038,068, a compound lens consisting of a pair of lenses made of materials different in dispersion from each other is provided in an optical path. The compound lens is rotated about the center of curvature of light entrance and exit surfaces thereof as the center of rotation, or moved in a direction perpendicular to the optical axis, to compensate the atmospheric dispersion.
The viewing angle diameter of the astronomical telescope using the principal focal point compensation optical system of U.S. Pat. No. 6,038,068 is 0.5 degrees. Recently, further improvement in survey performance of the astronomical telescope has been desired. To do this requires the principal focal point compensation optical system, which compensates aberrations caused by the primary mirror and chromatic aberration due to atmospheric dispersion, to widen the visual field of observation and improve optical performance.
Wider field of view and higher performance result in increase in the number of lenses that construct the principal focal point compensation optical system. As the number of lenses that construct the principal focal point compensation optical system increases, the principal focal point compensation optical system becomes heavier. In astronomical observation, if tracking observation of a star image located other than the zenith is carried out, a structure (lens barrel) supporting the primary mirror, the image pickup element, and the principal focal point compensation optical system tilts with respect to the zenith. The tilt results in self-weight deformation, and if this self-weight deformation causes a relative tilt between the image pickup element and the principal focal point compensation optical system to produce decentering aberration, the imaging performance is degraded.